The present invention relates to a process for case hardening martensitic stainless steels.
The chemical compositions of martensitic stainless steels are shown in the following Table 1.
TABLE 1 ______________________________________ TYPE WEIGHT % JIS AISI BS C Si Mn Ni Cr ______________________________________ 410 410 (En56B) &lt;0.15 &lt;1.00 &lt;1.00 &lt;0.60 11.50 .about.13.50 440A 440A -- 0.60 " " " 16.00 .about.0.75 .about.18.00 440C 440C -- 0.95 " " " 16.00 .about.1.20 .about.18.00 420F 420F En56D 0.26 " " " 12.00 .about.0.40 .about.14.00 ______________________________________
There are many recent examples in which stainless steels have been hardened by solution heat-treatment for use in machine parts, tools, and screws, etc. which tend to undergo undesirable corrosion. However, with conventional heat-treating methods, it is difficult to obtain bright surfaces and sufficient hardness. Furthermore, these methods involve a problem relating to the corrosion resistance of the surface layer. In particular, screws require both sufficient resistance against planar pressure by loads and sufficient strength to withstand their normal use.
Screws used for structures made of a non-ferric material such as aluminum or copper, or thin sheet steel are frequently used directly for tapping without drilling holes in the structures beforehand. While the screws used for this purpose are required to have high hardness and tensile strength, the conventional heat-treating methods have a disadvantage in that the screws obtained by such treatments do not have sufficient surface hardness.
In general, various treating methods such as carburizing, carbonitriding, and nitriding have been widely used heretofore to harden the surfaces of the usual iron steels as well as stainless steels. Stainless steel is a material which is difficult to carburize and nitride, and these prior surface-treating methods do not work satisfactorily because they cause the corrosion resistance of the stainless steel to deteriorate. With respect to the solution heat-treatment of martensitic stainless steels, therefore, it has long been desired to realize a hardening treatment without impairing their corrosion resistance.
As for the solution heat-treatment of martensitic stainless steels, what has generally and frequently been employed is a process comprising heating a martensitic stainless steel to 920.degree.-1070.degree. C. and then quenching it. The conventional treatment process, however, results in a poor surface condition and it is difficult to obtain sufficient hardness and high corrosion resistance.
As regards the nitrogen gas, it is theoretically known that molecular nitrogen dissociates into atomic nitrogen at high temperature. When molecular nitrogen directly dissociates into atomic nitrogen, dissociation occurs in accordance with the following formula (1). EQU N.sub.2 .fwdarw.2N+224.86 kcal . . . (1)
In other words, conversion of 1 mole of molecular nitrogen to atomic nitrogen requires an energy of 224.86 kcal. It is anticipated, therefore, that conversion to atomic nitrogen is extremely difficult.
In the presence of oxygen at high temperatures, nitrogen oxide is formed by the reaction defined by formula (2), and then atomic nitrogen is dissociated in accordance with formula (3). EQU N.sub.2 +O.sub.2 .fwdarw.2NO+43.2 kcal . . . (2) EQU NO.fwdarw.N+1/2O.sub.2 +90.85 kcal . . . (3)
It is known that in the process of forming nitrogen oxide by the combination of nitrogen and oxygen in the reaction of formula (2), the reaction proceeds at a very high temperature. But it is assumed that the rate of reaction is extremely slow, and the reaction scarcely proceeds even in the presence of a flame.
Atomic nitrogen is required in order to cause the adsorption and diffusion of nitrogen on the surface of the stainless steel to be treated. Usually, however, nitriding is extremely difficult by reactions based on formulae (1), (2) and (3), and therefore case hardening by the diffusion of nitrogen is extremely difficult.